US-South Korea Collaborative Research: Additive Manufacturing of Fatigue Resistant Materials

增材制造或 3D 打印能够制造传统上无法用于各种应用的定制复杂金属零件。 因此，工程设计和产品实现的范式转变正在发生，生物医学和航空航天等许多行业都将受益。 一些例子包括（1）现场快速制造具有针对患者和损伤的设计的金属骨植入物，以及（2）在偏远地区（例如外层空间）制造替换部件。 尽管如此，通过当前增材制造方法制造的金属零件往往具有孔隙率和各向异性，这些特征通常不利于零件强度和抗疲劳性。此类零件不能放心地用于承载应用。 该奖项支持科学研究，有可能通过增材制造生产抗疲劳金属零件。 本研究的目标是（1）建立微观结构特性（晶粒取向和形态）、孔隙率分布（尺寸、形状和位置）和工艺参数（激光功率、扫描速度、填充间距和层取向）之间的关系； (2) 了解增材制造材料的微观结构特性（晶粒取向）和孔隙率对其多轴疲劳抗力的影响。 为了实现第一个目标，将开发连续尺度热物理模型并用于将微观结构特性和孔隙率分布与工艺参数联系起来。 这些模型将得到实验验证。 Ti-6Al-4V 样品将在各种工艺参数组合下使用基于激光的增材制造技术来制造。 层方向将在 0°-90° 之间改变，而扫描速度、舱口间距和激光功率将在现有知识（例如已发表的实验数据）规定的范围内变化。将使用 X 射线断层扫描（孔隙的尺寸、形状和位置）以及光学和扫描电子显微镜（晶粒取向和形态）来测量制造样本的微观结构特性和孔隙率分布。 为了实现第二个目标，将开发基于临界平面方法的多轴微观结构敏感疲劳模型并通过实验进行验证。 将在制造的样本上进行多轴疲劳实验，使用同相和异相区分载荷路径来练习不同的临界载荷平面。 对样品断裂表面进行断口分析，以确定引发裂纹的孔隙的位置、尺寸和形状。将采用裂纹复制技术来查找疲劳微裂纹相对于临界载荷平面的方向，并确定各向异性微观结构对疲劳行为的影响。通过与韩国工业技术研究所的合作，生成的模型将在其他增材制造方法和材料上进行测试。

Additive manufacturing, or 3D printing, offers the ability to fabricate customized, complex metallic parts traditionally unobtainable for a variety of applications. A paradigm shift in engineering design and product realization is thus occurring, and many industries, such as biomedical and aerospace, are poised to benefit. Some examples include (1) on-site, rapid fabrication of metallic bone implants with patient and injury-specific designs, and (2) fabrication of replacement parts in remote locations (e.g. outer space). Nonetheless, metallic parts made by current additive manufacturing methods tend to have porosity and anisotropy, features typically detrimental to part strength and fatigue resistance. Such parts cannot be used with confidence in load-bearing applications. This award supports scientific investigation that can potentially enable production of fatigue resistant metallic parts by additive manufacturing. The objectives of this research are (1) to establish relationships between microstructure properties (grain orientation, and morphology), porosity distribution (size, shape, and location), and process parameters (laser power, scanning speed, hatch spacing, and layer orientation); and (2) to understand effects of microstructure properties (grain orientation) and porosity of additively-manufactured materials on their multi-axial fatigue resistance. To achieve the first objective, continuum-scale thermophysical models will be developed and used to relate microstructure properties and porosity distribution to process parameters. These models will be experimentally validated. Ti-6Al-4V specimens will be fabricated using laser-based additive manufacturing under various process parameter combinations. The layer orientation will be altered between 0º-90º, while scanning speed, hatch spacing, and laser power will be varied within ranges prescribed from existing knowledge (e.g. published experimental data). Microstructure properties and porosity distribution of fabricated specimens will be measured using X-ray tomography (size, shape, and location of porosity), as well as optical and scanning electron microscopy (grain orientation, and morphology). To achieve the second objective, multi-axial microstructure-sensitive fatigue models based on critical plane approaches will be developed and validated by experiments. Multi-axial fatigue experiments will be conducted on fabricated specimens, using in-phase and out-of-phase discriminating load paths to exercise different critical loading planes. Fractography on the fracture surface of specimens will be performed to determine location, size, and shape of the pore(s) responsible for initiating cracks. Crack replication techniques will be employed to find the orientation of fatigue micro-cracks with respect to critical loading plane and to determine effects of anisotropic microstructure on fatigue behavior. Through the collaboration with the Korea Institute of Industrial Technology, the generated models will be tested on other additive manufacturing methods and materials.